The invention relates to the fabrication of semiconductor devices, such as dynamic random access memory devices, and more particularly to patterning and fabrication surface p-channel transistor devices.
Complementary metal-oxide-semiconductor (CMOS) technology is widely used in integrated circuits (ICs) due to the lower power consumption as compared to previously preferred NMOS or Bipolar IC""s. CMOS is so named because it uses both p-and n-channel transistors in its ICs. However, fabricating both p-channel and n-channel transistors on the same IC adds a number of processing steps to the IC fabrication process.
The n-channel devices typically require n+ poly silicon gate material to set the correct turn-on voltage (threshold). The p-channel devices can be either a buried channel or surface channel device depending on whether n+ or p+ poly silicon is used for the gate material. When a p-channel transistor uses the same n+ poly silicon material as the NMOS device, then it is considered to be a buried p-channel device and the subsequent fabrication process becomes cost-effective. As a result, buried p-channel devices were predominantly used in DRAM and SRAM technologies. The disadvantage of buried p-channel devices is that they are slow (lower current drive) and have non-scalable threshold voltages. DRAM""s, till recently, decided to stay with the buried p-channel transistors since it kept the rest of the processing in the array and peripheral n-channel transistors simpler. On the other hand, to achieve higher speed and voltage scalability the SRAM""s elected to switch to surface p-channel devices. CMOS with surface p-channel transistors, however, needed a hardened gate oxide and two separate formations of gate poly silicon (n+ and p+) resulting in an extra masking step. With the addition of across the die hardened gate oxide, this surface p-channel CMOS design had to settle for slightly degraded n-channel transistors, because only one gate oxide deposition step was practical.
A prior fabrication method used in forming a pair of complementary surface channel CMOS transistors is as follows. First a P-well and an N-well is created in the substrate, then a gate oxide is formed over the complementary wells. To form the gates, an intrinsic poly silicon layer is deposited over the gate oxide. Then, a gate region for the p-channel device is masked over the N-well and the intrinsic poly silicon layer in the p-channel gate region is doped to P-type poly silicon. After doping the p-channel gate region, the mask is removed, and a gate region for the n-channel device is masked over the P-well and the intrinsic poly silicon layer in the n-channel gate region is doped to N-type poly silicon.
One drawback with this method of IC manufacture is that it requires several lithography masking steps. The substrate must be masked while forming the P-well. The substrate must be masked again when forming the N-well. Another masking step is needed to introduce the P-type dopant for the p-channel gate, and a fourth masking step is needed to introduce the N-type dopant for the n-channel gate. Lastly, a fifth masking step is needed to pattern the final gate stack just prior to the isolation step.
In doping the poly silicon gate layer, the dopant can be diffused into the layer by exposure to a dopant atmosphere, or the dopant can be ion implanted. In either case, the concentration profile through the thickness of the doped poly silicon layer varies significantly. This concentration gradient is detrimental to device performance. Gates formed by doping poly silicon tend to form a poly depletion layer near the gate oxide during operation, which causes an increase in effective gate oxide thickness. The dopants introduced to the poly silicon layer are more heavily concentrated in the top surface of the gate region than at the bottom, adjacent the gate oxide. An increase in effective gate oxide thickness can decrease device performance. Therefore, it is preferable to have in-situ doped poly silicon over implanted poly silicon.
Another major disadvantage of this approach, as seen by DRAM""s, is that one can no longer rely on conventional source-drain re-oxidation schemes to selectively increase the oxide thickness at the poly gate edges due to the presence of hardened gate oxide. The hardened gate oxide inhibits oxygen diffusion from beneath the gate, which is needed to increase the oxide thickness at the poly gate edges. The DRAM""s rely on this step to lower the electric field at the gate-edge and thereby reduce the gate-induced drain leakage (GIDL) in the access transistor.
What is needed is a method of gate formation in CMOS circuits that uses fewer masking steps while still avoiding poly depletion effects. What is also needed is a method of forming gates for CMOS surface channel transistors where the gate dielectrics may be individually tailored for composition and number of layers to perform optimally with the associated gate material.
An improved method of making CMOS surface channel transistors using fewer masking steps is shown. A single mask step may be used to mask a first gate poly silicon layer. The mask may then be used to form a doped well, to optionally remove a first dielectric layer and deposit a second dielectric layer, and to deposit a second gate poly silicon layer that is complementary to the first poly silicon layer. In-situ doped poly silicon deposition may also be used in both P-type and N-type poly silicon layers to reduce problems with poly depletion effects.
Using this method, the first and second gate dielectrics thicknesses and compositions can be separately controlled and the second gate dielectric is localized to areas that specifically need it. One doped well can be implanted through the first gate dielectric, and the first gate dielectric can then be removed without concern for any damage created during the implant step. A second gate dielectric, tailored to the gate material that will be formed on top of the second gate dielectric, may then be formed with a different number of layers, a different composition, and a different thickness than the first gate dielectric if desired.
This method also allows the use of a lithography mask normally used to fabricate buried channel devices for use in fabricating surface channel devices, thus saving the manufacture of an additional mask.
Although specific embodiments have been listed, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive. Combinations of the above embodiments, and other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention includes any other applications in which the above structures and fabrication methods are used. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.